1. Field of the Invention
The present invention relates to display units and methods for making the same. In particular, the present invention relates to a display unit including a luminescent device that has an organic layer between electrodes and relates to a method for making the same.
2. Description of the Related Art
A typical organic electroluminescent (EL) device based on electroluminescence of an organic material has an organic layer that includes an organic hole transport sublayer and an organic luminescent sublayer between an anode and a cathode. The organic EL device has attracted attention as a high-brightness luminescent device that can be driven by low DC voltage. The organic EL device has a structure including a substrate, a lower electrode functioning as the anode or cathode on the substrate, the organic layer including the luminescent sublayer on the lower electrode, and an upper electrode functioning as the cathode or anode. The light emerging from the organic layer is emitted through the substrate or the upper electrode.
In the production of such an organic EL device, the lower electrode is formed on the substrate by patterning, the organic layer including the luminescent sublayer is deposited on the lower electrode, and then the upper electrode is formed on the organic layer while insulation of the upper electrode from the lower electrode is being ensured.
In this method, each layer is formed, for example, by sputtering or vacuum evaporation. In the process, the upper electrode must be formed without damaging the underlying organic layer. If the organic layer is damaged, the luminescent efficiency to the amount of charge injected decreases. Thus, a large current must be applied between the electrodes for achieving high brightness. Unfortunately, a large current in the organic EL device accelerates deterioration of the organic layer, resulting in a decrease in lifetime (brightness half-life) and an increased in power consumption of the display unit.
In view of such circumstance, the upper electrode is generally formed by vacuum deposition which less damages the underlying organic layer. When the upper electrode is formed by sputtering, an organic buffer layer composed of, for example, copper phthalocyanine is provided on the organic layer, and the upper electrode is formed on the buffer layer to protect the organic layer during the sputtering process, as disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 2000-58266 and 2000-340364 and U.S. Pat. No. 6,172,459B1.
However, this method has the following disadvantages: The method requires an additional step for forming the buffer layer on the organic layer, resulting in an increase in production cost and a complicated display unit structure.
Furthermore, the buffer layer of the resulting display unit absorbs light of a specific wavelength range; hence, the display unit does not have desired light-emitting characteristics in this specific wavelength range. For example, the buffer layer composed of copper phthalocyanine has a large absorption peak in a red wavelength range, more specifically absorbs about 20% light even if the layer thickness is 14 nm. Thus, a red organic EL device exhibits a low light-emitting efficiency, in other words, low external quantum efficiency.
Such low external quantum efficiency results in an increase in current for achieving required brightness and a decrease in brightness half-life.
When the upper electrode is formed by vacuum evaporation, the deposition rate is unstable. Thus, the resulting upper electrode does not have a predetermined thickness. When the upper electrode is formed by co-evaporation using a plurality of materials, it is difficult to fix the composition. Thus, many idling hours are required for stabilizing the composition in the co-evaporation Furthermore, these materials must be supplied frequently. Accordingly, the vacuum evaporation process has reduced productivity.